HOW REPUBLIC’S EXPERIMENTAL TURBOPROP FIGHTER BECAME THE LOUDEST AIRCRAFT EVER BUILT
When the renowned designer Alexander Kartveli began work on the future P-84 based on a General Operational Requirement issued by the US Army Air Force on September 11, 1944, he could scarcely have imagined that between 1945 and 1958, Republic Aviation Corporation, with assistance from General Motors, would construct nearly 8,000 F-84s across all variants. This remarkable production figure was achieved thanks to manufacturing capabilities reaching 200 aircraft per month—a rate that would make today’s aerospace manufacturers envious. Of course, substantial evolution occurred between versions, with virtually nothing from the earliest models appearing on the later variants. The fuselage, cockpit, tail assemblies, wing configuration and powerplants were all extensively modified, with only the basic F-84 designation remaining unchanged—primarily for obscure budgetary presentation purposes.
Unlike its contemporary, the North American F-86 Sabre, which never served as a platform for experimental programs venturing beyond conventional fighter design, the F-84 became associated with several extraordinarily innovative projects that triggered further transformations. These included wing-tip mounted parasite fighters (Operation Tom-Tom) and reconnaissance aircraft carried within a B-36 bomber’s weapons bay (Operation Ficon). However, none proved as radical as the modifications applied to the XF-84H, which became the last “Thunder” variant to employ a propeller.
A SUPERSONIC PROPELLER
By the conclusion of World War II, turbojet development had enabled cutting-edge aircraft to achieve performance levels previously unattainable with piston-engine powerplants.
Since propeller efficiency during this era declined substantially at high Mach numbers, the technology appeared to have reached its practical limits, at least for combat aircraft. Naturally, aircraft manufacturers worldwide redirected their research toward pure jet designs, despite the significant challenges posed by unreliable and extremely fuel-hungry engines that forced designers to dramatically increase internal fuel capacity while simultaneously accepting greatly reduced operational range. In the United States, however, certain researchers—recognizing the inherent advantages of propellers—began investigating the feasibility of propeller systems capable of operating efficiently at speeds far exceeding those attainable with piston engines. Three principal concepts guided their research: first, designing blades with nearly perfect aerodynamic geometry; second, utilizing airfoil profiles as thin as physically possible; and third, operating the propeller at extremely high RPM so that the outboard sections of the blades would travel at supersonic speeds. This advanced propeller concept would necessarily be paired with a turboprop engine—a powerplant type that would prove exceptionally challenging to develop, particularly in the United States.
After the supersonic propeller’s potential had been demonstrated in National Advisory Committee for Aeronautics (NACA) wind tunnels, the next phase required testing under actual flight conditions using a high-performance aircraft. In July 1949, the US Air Force decided to modify the first McDonnell XF-88 Voodoo prototype by installing an Allison XT38-A-5 turboprop in its nose section. The approach involved utilizing an otherwise obsolete prototype as a flying test bed, which—thanks to its twin turbojet engines—could safely perform the experimental mission. In this modified configuration, it completed its maiden flight on April 14, 1953, and by April 24, the turboprop was successfully operated in flight for the first time. Parallel to this cautious and prudent approach—the XF-88B would ultimately serve NACA for more than four years—the Air Force issued a request for proposals in January 1950 seeking manufacturers to convert an existing jet aircraft to rely exclusively on turboprop power with a supersonic propeller. Following thorough evaluation of several proposals meeting Air Command technical requirements, the Air Research and Development Command awarded Contract No. AF 33(038)-20501 to Republic Aviation for converting two RF-84F-26-RE aircraft into XF-84H testbeds.
This selection was justified by the RF-84F’s solid nose section, which appeared ideally suited for turboprop installation with air intakes that could be readily adapted from those already positioned in the wings of the Thunderflash. However, Republic’s engineers quickly encountered formidable challenges because the airframe they had selected was never designed to withstand the tremendous torque generated by a high-speed propeller. Stability and flight control issues appeared insurmountable with the initial design parameters, and wind tunnel testing with powered models proved disastrous—the XF-84H demonstrated complete instability and uncontrollability. To address these critical deficiencies, extensive modifications were developed with Air Research and Development Command approval. Ultimately, the redesigned aircraft diverged so dramatically from the original concept that it warranted an entirely new designation: XF-106.
Only the canopy and wing section from the RF-84F remained in the final design. Why, then, maintain the XF-84H designation? During budget discussions, the Air Force strategically presented spending programs by aircraft type. It proved far simpler to secure funding for what appeared to be a straightforward F-84 modification—buried within the substantial Thunderstreak and Thunderflash procurement—than to attract scrutiny from vigilant lawmakers who might question an entirely new XF-106 fighter concept featuring what would have seemed like an anachronistic propeller system in 1951-1952. This administrative sleight-of-hand had precedent with the F-84F, whose prototype initially carried the YF-96 designation. However, virtually no components were shared between the straight-wing Thunderjet and swept-wing Thunderstreak.
~~~ To Be Continued ~~~
